1. Field of the Invention
The present invention relates to a power module in which an insulated circuit board is fixed to one main surface of a heat discharge plate.
This application is based on Patent Application Nos. 2000-240580 and 2001-194034 filed in Japan, the contents of which are incorporated herein by reference.
2. Background Art
As is shown in FIG. 5A, a power module 1 of this type is known in which an insulated circuit board 3 is fixed to one main surface of a heat discharge plate 2. The insulated circuit board 3 is manufactured by bonding first and second aluminum (Al) plates 3b and 3c to both surfaces of a ceramic substrate 3a. The second Al plate 3c of the insulated circuit board 3 is formed by etching as a circuit having a predetermined pattern, and a semiconductor chip 4 or the like shown by the dotted chain line is mounted thereon. The first Al plate 3b of the insulated circuit board 3 is bonded to a top surface of the heat discharge plate 2 that is formed of an aluminum based alloy via a stress cushioning layer 6 that is formed of an AlSiC composite material, and if required the heat discharge plate 2 can be mounted on a heat sink (not shown). In this conventional power module 1 heat generated by the semiconductor chip 4 or the like is transmitted to the heat discharge plate 2 via the second Al plate 3c, the ceramic substrate 3a, the first Al plate 3b, and the stress cushioning layer 6. Thereafter, this heat is dissipated from the heat discharge plate 2 or the unillustrated heat sink on which the heat discharge plate 2 is mounted.
In order for the heat generated by the mounted semiconductor chip 4 or the like to be dissipated in the power module 1, it is preferable that the distance between the semiconductor chip 4 or the like generating the heat and the heat discharge plate 2 that actually dissipates the heat is shortened, and the thermal resistance over this distance be reduced as much as possible. For this reason, as is shown in FIG. 5B, instead of providing the stress cushioning layer 6, fixing the insulated circuit board 3 directly to the heat discharge plate 2 may be considered.
However, if the insulated circuit board 3 is fixed directly to the heat discharge plate 2, then as a result of the temperature of the insulated circuit board 3 repeatedly changing from a high temperature to a low temperature and vice versa by the heat generation and non-heat generation of the semiconductor chip 4 or the like, the drawback occurs that, as a result of stress being repeatedly applied to the first Al plate 3b forming the insulated circuit board 3, the portion to which the stress is applied becomes work-hardened. Moreover, if the first Al plate 3b becomes work-hardened, then it becomes difficult for the stress to be absorbed, and the problems arise of peeling occurring in the bond portion and the heat cycle longevity of the power module 1 being shortened.
Furthermore, if the heat discharge plate 2 is fixed to the insulated circuit board 3 by brazing, it is generally necessary for the brazing to be performed at a comparatively high temperature, and the drawback occurs that when the completed brazing is cooled, marked warping is generated in the power module 1 at room temperature due to disparities in the thermal coefficient of contraction.